Technical Field
The present invention relates to a carbon-based composite material, and in particular, to a method for fabricating a carbon-based composite material.
Related Art
Persons skilled in the art all know that diamond is formed of sp3-bonding carbon. In addition, diamond and relevant materials thereof are widely used by the industry owing to outstanding physical and chemical properties thereof. Using physical properties of diamond as an example, it is advantageous to use diamond films as materials for fabricating emitters of field emission owing to excellent electron field emission (EFE in short) properties of the diamond films. On such basis, in recent ten years, reports relevant to research, development, discussion, etc. of carbon-based composite materials have constantly appeared in the industry.
The inventor disclosed a method for synthesizing a composite diamond film of microcrystalline diamond-ultrananocrystalline diamond (MCD-UNCD) in the article Enhanced electron field emission properties by tuning the microstructure of ultrananocrystalline diamond film (called earlier case 1 below) published in Journal of Applied Physics 109, 033711 (2011). The method for synthesizing a composite diamond film of the earlier case 1 is first performing ultrasonic agitation on a plurality of n-type (100) silicon substrates separately in a solution containing diamond powder with a grain size of about 1 nm for 30 min, and then performing ultrasonic cleaning by using acetone to remove fine grains adsorbed on each of the n-type silicon substrates; next, depositing each of the cleaned n-type silicon substrates in a first plasma atmosphere containing argon (Ar) and methane (called CH4 below, 2%) for 60 min by means of microwave plasma enhanced chemical vapor deposition (MPECVD), so as to form a UNCD seeding layer on a surface of each of the n-type silicon substrates, where the UNCD seeding layer is grown on an amorphous carbon layer with a thickness of about 1 nm, and is formed by UNCD grains, a grain size of which is about 5 nm, a grain boundary of which is an amorphous carbon matrix, and a thickness of which is about 300-1000 nm; and finally, depositing, also by means of MPECVD, each n-type silicon substrate with the UNCD diamond seeding layer formed on the surface thereof in a second plasma atmosphere containing CH4/Ar/hydrogen (call H2 below) in a ratio of 1/(99−x)/x (where x is 0, 25, 50, 75, and 90) for 30 min, 60 min, or 90 min, so as to form an MCD layer on each UNCD diamond seeding layer.
The research of the earlier case 1 points out that by means of the synthesis method, the composite diamond film obtained by performing deposition for 60 min under the condition that the ratio of CH4/Ar/H2 in the second plasma atmosphere is 1/49/50 can obtain the optimal EFE property; that is, a turn on electric field (called E0 below) of 6.5 V/μm, and a current density (called J below) reaches 1 mA/cm2 under the condition that an applied electric field (called Ea below) is 30 V/μm.
Further, the inventor disclosed a method for growing an ultrananocrystalline diamond film (called UNCD below) having a needle-like granular structure in the article Origin of a needle-like granular structure for ultrananocrystalline diamond films grown in a N2/CH4 plasma (called earlier case 2) published in J. Phy. D: Appl. Phys. 45 (2012) 365303 (9pp). The growth method of the earlier case 2 is first performing ultrasonic agitation on a plurality of n-type (100) silicon substrates separately in a solution containing diamond powder with a grain size of about 30 nm, titanium (Ti) powder with a screen size of 325 nm, and methanol for 45 min, so as to generate a plurality of nucleation sites on a surface of each of the n-type silicon substrates; next, providing each of the n-type silicon substrates in an MPECVD system (IPLAS-CYRANNUS, 2.45 GHz) to heat the surface thereof to deposition temperatures such as 550° C., 600° C., 650° C., 700° C., 750° C., and 850° C., and introducing a mixed gas containing nitrogen (called N2 below; 94%) and CH4 (6%) into the MPECVD system; and generating a microwave plasma under a working pressure of 50 Torr at the microwave output power of 1200 W, so as to grow a UNCD film on the surface of each of the n-type silicon substrates.
The research of the earlier case 2 points out that at a suitable deposition temperature, a CN species in the microwave plasma used in each process of implementing MPECVD is advantageous to anisotropic grain-growth of ultrananocrystalline diamond in each UNCD film, so as to form nano needle-like diamond grains, and the nano needle-like diamond grains are wrapped by a nano graphite phase, so as to improve the EFE property and various electrical properties of each UNCD film. The EFE property and various electrical properties of the earlier case 2 are simply listed in the following table 1.
TABLE 1Depositiontemperature (° C.)Conductivity (S/cm)E0 (V/μm)J (mA/cm2)5501.213.022.3860010611.642.766501477.143.157001866.133.36@7501109.122.838009013.711.99@is obtained when intensity of the applied electric field (Ea) is 8.8 V/μm.
According to table 1, it can be known that an excessively low deposition temperature (such as 550° C.) or an excessively high temperature (such as 800° C.) both hinder anisotropic grain-growth, and make the turn on electric field (E0) thereof increase to 13.02 V/μm and 13.71 V/μm respectively; in addition, the conductivity (called σ below) of the UNCD film obtained by growth at the deposition temperature of 550° C. is only 1.2 S/cm. The turn on electric field (E0) and current density (J) of the UNCD film obtained by growth under the condition that the deposition temperature is controlled at 700° C. may decrease to 6.13 V/μm and increase to 3.36 mA/cm2 respectively, and the conductivity (σ) may increase to 186 S/cm.
By means of the growth method disclosed in the earlier case 2, the CN species in the microwave plasma used in the process of implementing MPECVD can assist in anisotropic growth of UNCD grains into nano needle-like diamond grains, and the EFE property and electrical properties of the UNCD film thereof can be adjusted by means of different deposition temperatures. However, in the earlier case 2, the deposition temperature at which the optimal EFE property and electrical properties are obtained reaches up to 700° C.
According to the foregoing description, it can be known that constantly looking for different methods for fabricating a carbon-based composite material to improve an electron field emission (EFE) property and improve electrical properties thereof is a difficult problem to be overcome by persons skilled in the art.